Automatic control system for carrier-wave signal beceivers



N0 49 1.941 R. L. FREEMAN 2926159@ AUTOMATIC CONTROL SYSTEM FORCARRIERWAVE SIGNAL RECEIVERS Filed Sept. 13, 1939 ATTORN EY tunedwithout losing the signal.

Patented Nov. 4, 194i iris sara AUTO'EFC CONTROL SYSTEM FOR, CAE-RiER-WAVE SHGNAL'RECEIVERS Robert L; man, Flushing, N. Y., assigner to yHazeltine Corporation, a corporation oi Deia= WSE@ assuma@ september is,was, sensi no. erases i comme (ci. ases-ae) action should also beindependent of signalI strength. The frequency interval over which theaction is effective should be chosen as a compromise between twolimitations; the control ac tion should be sufficient to correct foroscillator frequency instability caused by temperature and Y voltagechanges and for mistuning caused by shortcomings in mechanical presettuning devices; and in theother extreme, the. control action should beless than that which would he operative by the carrier signal on anadjacent channel. In a broadcast receiver this compromise suggests achoice 'of automatic frequency control action over a frequency range oiabout 'l kilocycles on either side of the desired signalcarrierfrequency. Thus, with conventional automatic frequency control, thesensitivity char acteristic or S-curve of the discriminator must fextend over a given frequency range of about 14 kilocycles. Attempts toobtain an S-curve of large slope sensitivity and consequently finertuning correction generally are frustrated because any `increase in theslope of such characteristic S-curve tol a certain extent reduces thegiven frequency range to one smaller than the permissible ifikilocycles.

As pointed out in the article entitled "Improvements in A. F. C.Circuits, pp. 20-23 of the November i936 issue of Electronics thefrequency separation oi the S-curve peaks determines to some extent theamount of mistuning of a signal for which the automatic-frequencycontrol system will lock in. Once this locking has occurred, thecontrol-tube sensitivity determines how much the local oscillator may bemis- For a satisfactory locking-in, the peak separation on the S-curveof the discriminator should not be much greater than the receiverover-all band width at twice resonant voltage input as otherwise the'band width of the intermediate-frequency amplifiers preceding thediscriminator network de termines the amount of mistuning of a signalfor which the automatic frequency control system will loci: in. Thislimitation becomes severe and decreases the value of an automaticfrequency control system whenever the deviation of the oscillatorfrequency from that which produces the correct intermediate frequency islarger than half the band width of the intermediate-frequency amplifierpreceding the' discriminator network. Such condition is sometimespresent in highly-selective broadcast receiversusing preset tuningmechanisms and is quite generally present in ultra-high-frequencyreceivers used for television reception or for frequency-modulated wavereception.

in the case of a television s uperheterodyne receiver, normal changes intemperature alone cause a frequency drift of the local oscillator ofseveral hundred kilocycles. Because of certain advantages as tosimplicity and economy, it is preferable to utilize the accompanyinglsound carrier to a television signal as the pilot signal to operate theautomatic frequency control system. Thus. the discriminator network isplaced in the intermediate-frequency amplifier channel which isresponsive to the sound signal-carrier frequency. From the standpoint ofreducing noise and interference itis desirable to make the band width ofthis sound amplifier channel no greater than necessary to accept thesound signal sidebands; in consideration of this, a band width of lessthan 50 kilocycles is desired. Suchl a choice is directly in conflictwith the large band width required to give satisfactory automaticfrequency control.

It is an object of the invention, therefore, to provide an improvedautomatic control system for a carrier-wave signal receiver whichovercomes the above-mentioned disadvantages of such arrangements of theprior art.

It is a further object of the invention to provide an improved automaticfrequency control system for superheterodyne receivers which hasrelatively high sensitivity and high tuning correction eiciency andwhich, at the same time, operates automatically from an amplier channelhaving a. relatively narrow frequency range without decreasing thepermissible frequency tolerance for manual or automatic tuning or foroscillator frequency instability.

In accordance with the invention. there is provided an automatic controlsystem for a carrierwave signal receiver comprising a network the outputof which varies in response to variation of the frequency or other givencharacteristic of a signal input thereto over a given range of variationof the frequency or other characteristic,

i e teristic of the signal input to the network over a range which iswider than and which overlaps the given range. The system also includesmeans responsive to the resulting frequency or other characteristic ofthe signal input to the network for rendering the additional meansinoperative when the resulting frequency or other' A characteristic hasa value within the given range.

In a preferred embodiment of the invention as applied to asuperheterodyne receiver, the intermediate-requency signal is applied tothe frequency discriminator, and the additional means varies thefrequency of the intermediate-irequency signal input to thediscriminator so that it sweeps over a frequency range wider than, butwhich overlaps, the given frequency range ofthe discriminator network.The additional means eiectively applies to the oscillator control tube arelatively low-frequency sweeping or searching voltage whereby thereceiver is tuned automatically to signals which initially are out ofthe given frequency-response range of the discriminator. Also, inaccordance with a preferred 'embodiment of the invention, means areprovided for rendering the additional means inoperative when thefrequency of the signal input to the network is momentarily swept withinsaid given frequency range, which means comprises a vacuum tubecontrolled by the output of the discriminator and controlling theapplication of the sweep voltage to the oscillator control tube.

The novel features which are believed to be characteristic of thisinvention are set forth with particularity in the appended claims. Theinvention itself, however, both as to its organization and method ofoperation, together with' further objects thereof, will be bestunderstood by reference to the specification, taken in connection withthe accompanying drawing, in which Fig. 1 is a circuit diagram, partlyschematic, ofy a complete superheterodyne receiver including anautomatic frequency control system embodying the present invention; andFigs. 2

and 3 are circuit diagrams o different embodiments of automaticfrequency control systems constructed in accordance with the invention.

Referring to Fig. `1 of the drawing, there is shown a circuit diagram,partly schematic, of a complete modulated-carrier signal receiver of thesuperheterodyne type embodying the invention. This receiver comprises,in cascade, an antenna-ground circuit it, ii, a radio-frequency amplieri2, a modulator i3 and associated oscillator 22, intermediate-frequencyampliiiers it and i5, a frequency-discriminator and detector network it,an audio-frequency amplier i'i of one or more stages, and a soundreproducer i8.

general, a conventional superheterodyne receiver r including automaticvolume control and automatic frequency controlsystems, the operation ofwhich, in general, is well understood by those skilled in the art. Anyof a number of signals .1

intercepted by antenna-ground circuit it, it is selected and amplifiedin radio-frequency amplifier l2 and translated to oscillator-modulator22, i3, wherein it is converted to. an intermediatefrequency signal. Theintermediate-frequency signal is selectively amplied inintermediatefrequency amplifiers iii vand l5 and delivered to thefrequency-discriminator and detector network l, wherein theaudio-frequency signals and the automatic amplincation control andautomatic frequency control biasing potentials are derived. Theaudio-frequency signals, in turn, are amplified by audio-frequencyamplifier il and supplied to sound reproducer it for reproduction. Theautomatic amplication control bias derived from unit Wis effective tocontrol the amplification of one or more of the units I2, til, it, andi5 to maintain the amplitude of the signal input to the unit 'i6 withina relatively narrow range for a wide range of received signalintensities, while the automatic frequency control bias derived from theunit i6 is effective to control the frequency-control tube 2| tomaintain the frequency of the signal input to the network i@ at asubstantially constant value.

The detector and Irequency-discriminator network it comprises a primarycircuit including an inductance 2t tuned by a condenser 23.

and inductively coupled to an inductance 25 in a secondary circuit tunedby a vcondenser 26. Circuits 23, 2e and 25, 25 are tuned to the normalintermediate-carrier frequency of the receiver. Network it also includesdiode rectiilers 21 and 23, to which are applied the sum and difference,

respectively, of the voltages'across the primary circuit 2.7i, 2t andacross half of the secondary circuit 25, 28. To this end, a couplingcondenser 3i is connected between the high alternatingpotential side ofthe circuit 23, 2d and a mid-tap on secondary inductance 25, while themid-tap on inductance 25 is coupled by way of conductor le to a commonterminal B of individual load resistors 29 and t@ of rectiflers 21 and28, respectively. A by-pass condenser 32 is connected across loadresistors 29 and 3D in series.

The value of condenser 3i has a very marked eiect on the selectivity ofthe network i6 as to audio-frequency characteristics and also as todiscriminator characteristics. When the capacitance of such condenser 3iapproaches zero or is very small, the discriminator action lof thenetwork l@ disappears', but the audio-frequency fidelity is as good asconventional intermediatefrequency diode detector stages. However, acompromise value for the capacitance of the condenser 3i can be foundwhich gives Very good discriminator action as well as goodaudio-:Erequency delity. For an intermediate frequency of 450 kilocyclesand with a value of capacitance of condenser3i ln the range of 4 to 10auf, and with comparatively loose coupling of the'inductances 24 and 25,an automatic frequency control characteristic having a peak separationof 2 to 4 kilocycles is obtainable with a slope sensitivity of 30 to 40volts per kilocycle (per peak volt applied to the preceding amplier tubegrid) and with correspondingly high gain at the peaks. It will beappreciated that this is from 2 to 10 times the sensitivity ordinarilyutilized in prior practice in which a peak separation of the order of 14kilocycles is desired. However, if such a discriminator is used, it willnot produce automatic frequency control action until the receiver istuned almost to the frequency of the desired signal, that is to say, towithin 4 to 5 kilocycles from resonance. Thus, it will be seen thatnetwork it has a unidirectional-voltage output which varies in responseto variation of the frequency of the signal input to the network onlyover a given frequency range which is relatively small, preferably about2 kllocycles. The attainment of such a narrow range oi response of thediscriminator isfurther facilitated by inserting a resistor in serieswith lead I9 in con'el formance with the ideas set forth in theapplicants copending application Serial No. 230,820, tiled September 20,1938, which issued as Patent No. 2,184,072, December 19, 1939.

The oscillator control tube 2i and the'oscillator-modulator stage 22, i3constitute means responsive to the output of the network it for.adjustingl the frequency of the signal input thereto to a frequencywithin a desired small range, or maintaining the frequency of the signalinput thereto approximately at a predetermined value, within the givenrange of permissible frequency deviation when the frequency of thesignal input is within the given frequency range. That is to say, theoperation of the circuit described in detail up to this point, ingeneral, is similar to a conventional automatic frequency control systemexcept that an extremely sharp discriminator is used having asensitivity from 2 to l0 times the sensitivity of the usualdiscriminator and a much smaller separation between peaks oi thediscriminator characteristic s-curve.

However, according to this invention, there is provided additional meansfor periodically sweeping the frequency of the signal input to thenetwork it over a frequency range which is wider than and which overlapsthe normal or given frequency range of the discriminator and at a sweepvfrequency lower than any frequency within such given frequency range,together with means for rendering the additional. means inoperative,`when the frequency of the signal input to the network it is within suchgiven frequency. The additional means preferably comprises rneans foreftectively applying a low-frequency alternating potential to thecontrol tube for varying periodically the oscillator frequency;specifically', this meanscomprises a periodic searching potentialcircuit tid inductively coupled by means of a transformer iid, SE to theinput circuit E@ of the oscillator control tube 2i. The circuit 33comprises the output circuit of a vacuum tube $3 the control electrodeof which is applied a periodic voltage from a suitable source, such asconventional S0-cycle alternating current supply circuit te, over aconductor t. A filter network, comprising series resistors t, 50 and ashunt condenser 5i, preferably is coupled between the sup- I 3 plycircuit sa and the discriminar network ls to prevent the sweep voltageAfrom getting back into the automatic amplification control andaudio-frequency system.

The application of a periodic voltage to the control electrode of theoscillator control tube 2i causes the oscillator frequency to be sweptthrough a range which is wider thanbut includes the normal frequencyrange of the discriminator i6 and thus provides means' for augmentingthe action of the sharp discrlminator it. When the receiver produces anintermediate-frequency signal of the proper frequency as it is tuned inthe vicinity of the' desired broadcast signal and 'the oscillator '22 isswept through a relatively wide range as described above, the sharpdiscriminator takes eiect and locks in the signal in a conventionalmanner. as the discriminator action takes eiect, the diode rectifier 2&3develops a neglative-bias voltage across its load resistor tu, re-

gardless oi' the side from which resonance is approached, which operatesto* bias the control electrode of the vacuum tube tu to cui/on which,

in turn, renders inoperative the frequency sweep system of theoscillator 2i. The diode 2S and its associated circuit comprises 'meansresponsive to the resulting frequency of the signal input to thediscrimlnator network for rendering the additional or frequency sweepingmeans inoperative when the resulting frequency has a value within thegiven range of the discriminator. Such action alone provides limitedautomatic frequency control action and, with a very sharp discriminator,the range of the automatic frequency control action depends only on theamplitude of the periodic sweep voltage and not at all on signalstrength for signals above a certain threshold level.

summarizing the general features and operation of this system, anextremely sharp discriminatur network is used; then, in order thatsignais may be automatically tuned in, which are initially out of thenormal frequency-response range of the receiver dscriminator circuit, an

l, auxiliary periodic voltage effectively is added to the control tubebias in addition to any unldirectional-bias potential developed by thediscriminator. Thus, for (a-tune signals -that would otherwise beineuective to develop a discriminator bias, the system pulls in to thembecause the control tube is sweeping the signal input to the networkthrough a relatively wide frequency range. if the oil-tune signal is inthis swept frequency range, discriminatori' voltages are 'developed.each time the oscillator. traverses the frequency that causes thecorrect intermediate frequency to be developed.

Referring to Fig. 2 of the drawing, there is shown an automaticfrequency control system embodying a modiiied .formof the inventionwhich, in general, is similar to that described above ,in connectionwith Fig. l, similar elements of the two circuits having identicalreference numerals. In this system the diodes il', 29 of thediscriminator i@ are reversed so that a positive bias is developed atthe common terminal B oi' the resistors il@ and d@ of the networkforbiasing the control electrode of a gaseous vacuum-tube relay, such as a"Ihyratron, @il coupled to the circuit 3d. With no bias on the controlelectrode oi' the relay du, the tube'is nonconductive and a periodicsweep voltage appears at the transformer primary tit. When the controlgrid is made positive by the discnator bias developed by a signal lockedin, the relay ill yactsas aconductor and vproduction of interchannelnoises.

sweep voltage in the output circuit 83 of tube 3G.

'Such an arrangement is shown vin Fig. 3, in which the sweep voltageoutput of tube 36 is rectied by a diode rectifier il and the resultingbias voltage applied negatively over conductor 62 to thecontrol-electrode of an audio-frequency stage tube to bias tle latter tocutod and thus suppress re- The diode il and its associated circuit thuscomprises means responsive to the additional or frequency sweeping meansfor restricting the output of the receiver while the additional means isoperative. When a signal is tuned in, this suppressor bias completelyvanishes. In the circuit 'of Fig. 3, the transformer secondary 35 iscoupled to the cathode of control tube 2i and the supply circuit i3dpreferably is coupled in series with the cathode-biasing resistor it ofvacuum tube tt. This arrangement eliminates the need of afilternetwork,such as that comprising resistors liti, E@ and condenserEll of Fig. l, for preventing the sweep voltage from aecting theautomatic amplification control and audio-frequency chel of the lreceiver; otherwise, the systemsare substantially similar.

It will be understood that other types of frequency-discriminatornetworks may be utilized in place of the discriminator networkillustrated and described.

While there have been described what are at present considered to be thepreferred embodiments of this invention, it will be obvious to thoseskilled in the art that various changes and modifications may be madetherein without departing from the invention, and it is, therefore,aimed in the appended claims to cover all such changes and modicationsas fall within the true spirit and scope of the invention.

What is claimed is:

l.. In a carrier-wave signal receiver, an automatic control systemcomprising, a network the output of which varies in response tovariation of a given characteristic of a signal input thereto overagiven range of variation of said characteristic, means responsive tothe output of said network for maintaining said given characteristic ofsaid signal input to said network approximately at a predetermined valuewithin said given range of variation when said characteristic of saidinput wave has any value within said given range, additional means forperiodically sweeping the value of said characteristic. of said signalinput over a range which is wider than and which overlaps said givenrange, and means responsive to the resulting characteristic of thesignal input to said network for rendering said additional meansinoperative when said resulting characteristic has a value within saidgiven range.

2. An automatic frequency control comprising, a discriminator networkthe output of which varies in response to variation of the frequency ofa signal input thereto over a given frequency range, means responsive tothe outputv of said network for maintaining the frequency of said signalinput to said network approximately at a predetermined frequency withinsaid given frequency range when said frequency of said sigsystem 4assises nal input falls within said given frequency rang, additionalmeans for periodically varying the frequency of said signal input overa. frequency range which is wider than and which overlaps said givenfrequency range, and means responsive y to the resulting frequency ofthe signal input to said' network for rendering said additional meansinoperative when said resulting frequency has a value within said givenfrequency range.

3. An automatic frequency control system comprising, a discriminatornetwork the output of which varies in response to variation of thefrequency of a signal input thereto over a given frequency range, meansresponsive to the output of said network for adjusting the frequency ofsaid signal input to said network to a frequency within a desired smallfrequency range within said given frequency range when said frequency ofsaid signal input is within said given frequency range, additional meansfor periodically varying the frequency of said signal input so th'. t itsweeps at a frequency lower than any frequency within said givenfrequency range and over a frequency range which is wider than and whichoverlaps said given frequency range, and means responsive to theresulting frequency rof the signal input to said network for renderingsaid additional means inoperative when said resulting frequency has avalue within said given frequency range.

4. An automatic frequency control system comprising, means including adiscriminator network the output of which depends upon the frequency ofa signal input thereto over a given frequency, control means responsiveto said output for maintaining the frequency of .said signal inputapproximately Aat a predetermined frequency range within said givenfrequency range when the frequency of said input. wave falls within saidgiven frequency range, additional means comprising a searching potentialcircuit for varying periodically'the frequency of said signal input at afrequency lower than any frequency within said given frequency range andover a frequency range which is wider than and which overlaps said givenfrequency range, and means responsive to the output of saiddiscriminator for rendering said additional means ineffective when theresulting frequency of said signal input falls within said givenfrequency range.

5. In a carrier-wave signal receiver of the superheterodyne type, anautomatic frequency control system comprising, a discriminator network,an oscillator control tube, means including said discriminator networkfor developing a frequency-control bias and applying it to said tube,additional means for effectively applying a low-frequencyalternatingpotential to said control`tube for varying periodically the oscillatorfrequency, whereby automatically to tune the receiver to signals whichinitially are out of the frequency-response range of the discriminator,and means responsive to the resulting frequency of the signal input tosaid network for removing said low-frequency voltage from said controltube when said resulting frequency has any value within thefrequency-response range of said discriminator.

6. In a carrier-wave signal receiver of the superheterodyne type, anautomatic frequency control system comprising, an exceedingly sharpdiscriminator network the output of which varies in response tovariation of the frequency of an intermediate-carrier signal inputthereto over a given frequency range, an oscillator control tube, meansresponsive to said output for controlling said ccyitrol tube to adjustthe frequency of said intermediate-carrier signal input to a frequencywithin a desired small frequency range when said frequency of saidintermediate-carrier signal input has any value within said givenfrequency range, additional means for applying a low-frequencyalternating potential to said oscillator control tube for varyingperiodically the oscillator frequency so that it sweeps at a quencyrange, additional means for varying the frequency lower than theintermediate-carrier frequency of said receiver and overa frequencyrange which overlaps and is wider than said given frequency range, andmeans responsive to the output of said discriminator network forrendering said additional means inoperative when the resulting frequencyof said intermediate-carrier signal input falls within said givenfrequency range. I

7. In a modulated-carrier wave signal receiver, an automatic frequencycontrol system comprising, a discrlminator network the output offrequency of said signal input over a frequency range'which is widerthan and which overlaps said given frequency range, means for renderingsaid additional means inoperative when the resulting frequency of thesignal input to said network fallsY within said given frequency range,and means responsive to said additional means for restricting the outputAof the receiver while saidadditional means is operative, whereby tosuppress intercarrier noises when the receiver is not tuned to acarrier-wave signal.

ROBERT L. FREEMAN.

DISCLAIMER A 2,261,800.Ro1bert L. Freeman. Flushing, N. Y. A'ronA'ncCoNTnoL SYSTEM Fon CARRIER-WAVE SIGNAL Rncmvmns. Patent datedNovember-1, 1941. Dis-v claimr filed September v2, 1943, by theinventor; assignee, Hazeltine -.(J'orporatwn, assent1ng. I Hereby entersthis disclaimer' to claims 4, 5, and 6 of said patent.

[Oficial Gaztte Septembr 28, 1943.]

DISCLAIMER 2,261,800.-Robert L. Freeman, Flushing, N. Y. AUTOMATICCONTROL SYSTEM FOR CARRIER-WAVE SIGNAL RECEIVERS. Patent; dated Novemberv4, 1941. Disclaimer filed June 22, 1944, by the inventor; the assignee,Hazeltne Corporation, assenting. Hereby enters this disclaimer to claims1, 2, and 3 of said patent.

v [Oficial Gazette August 1, 1.944.]

